As used in this specification, a colorant is any material that can impart color to the medium in which it is used. The colorant may be insoluble or soluble in the medium. Generally, colorants may be divided into two broad categories; pigments and dyes. As used in this specification, a pigment may be organic or inorganic, natural or synthetic, and is generally insoluble in the medium in which it is used, while a dye is generally soluble in the medium in which it is used.
As used in this specification, a subcategory inclusive of pigments are pigments used to modify the optical characteristics of the medium in which they are used, such as light reflectivity, light refractivity, light absorbance, or light transmittance. A subcategory of pigments used to modify the optical characteristics of the medium in which they are used are light-reflective pigments. Light-reflective pigments include, but are not limited to, metallic pigments (also known as metallic-flake pigments) and glitter (also known as flitter or spangles). A metallic pigment is a particle or flake, generally of metal or alloy, used to modify the optical characteristics of the medium, such as reflectivity, or to achieve a metallic appearance. The metals most commonly used include, but are not limited to, gold, silver, aluminum, copper, bronze, and zinc. Materials other than metals which may be used to achieve a metallic appearance include minerals (e.g. mica) or glass. These materials generally are all used in flake form.
Typically, products molded from plastic materials include, but are not limited to, automotive body panels, fascias and claddings. In the automotive industry, a preferred method of forming such products is injection molding of a plastic material, preferably that of a thermoplastic composition. Injection molding is preferred over other plastic forming techniques given its combination of high volume production with relatively low cost materials.
After molding, or post-molding, often a metallic appearance is applied to such automotive products in the form of a coating. Typically, this metallic appearance is achieved via a painting operation via the use of a metallic paint. A metallic paint is a paint which incorporates the use of a metallic pigment defined above.
Recently, there has been a desire to eliminate the post-molding application of a metallic appearance, such as via metallic paint, to such automotive products. There has been a desire to incorporate the metallic appearance for such automotive products during the forming, or molding, of the product itself. The incorporation of the metallic appearance for such automotive products during the forming of the product is desired to reduce cost, and the emission of potentially harmful volatile organic compounds (VOCs) into the atmosphere common to the painting process.
More particularly, there is a desire to incorporate the metallic appearance for such automotive products into the composition used for forming, or molding, the automotive product itself. There has also been a desire that the composition of the automotive product comprise a plastic material, preferably a thermoplastic material. Furthermore, there also has been a desire to form the above preferred thermoplastic material by a molding process, and, more particularly, an injection molding process.
More recently, attempts have been made to incorporate a metallic appearance into automotive products via thermoplastic injection molding. It has been found, through experimentation, that an acceptable level of quality of the molded automotive product can be achieved with an injection molding process and thermoplastic compositional alterations. Such is demonstrated by the 1999 Jeep Grand Cherokee Laredo front fascia, rear fascia and claddings, manufactured using an injection molding process with Solvay Engineered Polymers' Sequel 1440 grade of thermoplastic olefin (TPO) composition.
However, it has also been found that injection molding process and compositional-alterations required to produce an acceptable level of quality may vary significantly with each particular thermoplastic composition employed. More specifically, it has been found that, when different metallic pigments are used to achieve various metallic appearances and/or colors, the injection molding process and compositional alterations required to produce an acceptable level of quality between compositions may vary significantly. It has also been found that, in some cases, that even altering the injection molding process and thermoplastic composition, within acceptable limits, still may not produce an acceptable level of quality. Typically, such products did not produce an acceptable level of quality due to the presence of a differing metallic appearance on different locations on the surface of the product. This differing metallic appearance most noticeably appears in areas of the surface occupied by weld lines versus those areas of the surface without weld lines. Weld lines (also known as knit lines, flow lines, or weld marks) in a molded plastic product mark the meeting of different plastic flow fronts within a mold. According to Whittington's Dictionary of Plastics, 3rd Edition, the flow fronts may have cooled and skinned over before meeting, or had too little time in the molten state for interdiffusion of molecular segments across the interface.
As a result of the above difficulties, apparatus alterations, in addition to injection molding process and thermoplastic composition alterations, were the subject of experimentation. Specifically, apparatus alterations focused on alterations to the injection mold used for the injection molding process. More specifically, injection mold alterations focused on gate design, such as size, configuration, and location. In injection molding, the gate is the channel through which molten plastic flows from the runner into the mold cavity.